In many situations it is desirable to be able to monitor changes in stress in substantial masses of material. For example, in underground mines, as material is mined out to create a tunnel or other opening, a low pressure region is created in the mined material adjacent the wall of the opening, by reason of removal of the mined material. Stress levels in the mass being mined, will tend to equilibriate, causing changes in the stress levels at various points in the mass. If the stress changes are severe enough, failure of the mass is likely to occur, in reaching a new equilibrium state. Such failures are often referred to as "rock bursts".
In the past, various methods have been tried to attempt to predict the likelihood of a "rock burst" occurring. One method was to monitor the noise level in the rock as the stress pattern was changing. However, to date, it has proved very difficult to usefully correlate the noise level in the rock mass to the likelihood of a rock burst occurring. Another way, has been through the use of a borehole sensor unit, commonly known as the United States Bureau of Mines type of Borehole Deformation Guage (USBM gauge). However, the USBM guage has several severe limitations.
Essentially the USBM guage consists of a probe which is inserted into a borehole. Three pairs of strain-guaged cantilevers are mounted in a water-proof casing, to sense deformation of the borehole walls. The testing procedure consists of drilling an access hole, with a typical diameter of about 6 inches to the depth where testing is required. Then, a co-axial hole 1.5 inches in diameter is drilled, and the USBM gauge is inserted therein in a known orientation. Then, the co-axial 1.5 inches in diameter hole is over bored, typically with a 6 inch diameter overcoring cut. Then, the overcored material is removed, and the change in diameter due to relaxation of the borehole is measured when the core is removed from the host rock. Calculations are then made, based on the one time deformation recorded by the USBM gauge about the level of static stress present in the formation being tested.
However, the above method of measurement has several major disadvantages. Firstly, the testing required is very expensive and time consuming. For each test, three separate drilling operations are required to make a 6 inch hole, a 1.5 inch hole then a 6 inch overcore cut. Further, once the overcoring operation is complete, the rock tube containing the USBM gauge is removed. Therefore, the method only provides for a one time measurement of stress in the rock; it cannot, without conducting further tests, provide analysis of how the stresses in the rock are changing over time, as a result of mining operations. A number of tests are required over time to produce a statistically meaningful profile to changes in the static stress levels. Such a testing technique also requires a good deal of manpower and time and is expensive.
Further, dynamic stress levels, such as may occur as a result of underground blasting, or repetitive loading for example in a concrete bridge footing, or the movement of heavy machinery underground, cannot be sensed, as the USBM gauge is removed from the rock or other material to be tested. Dynamic loading can create stress excursions in the material which may, in certain instances, exceed the strength of the rock mass, and hence trigger a rock burst.
What is desired therefore is a material stress monitor that is easy to install with a minimum of drilling operations being required. What is also desired, is a material stress monitor, that will provide a real time record of changes in stress in the material being monitored, both in respect of static and dynamic loading, so as to help in the prediction of the likelihood of a rock burst occurring or to determine for example the timing of blasting delay periods.